Useful information on External Gear Pumps

A gear pump is a type of positive displacement (PD) pump. Gear pumps use the actions of rotating cogs or gears to transfer fluids.  The rotating gears develop a liquid seal with the pump casing and create a vacuum at the pump inlet.  Fluid, drawn into the pump, is enclosed within the cavities of the rotating gears and transferred to the discharge.  A gear pump delivers a smooth pulse-free flow proportional to the rotational speed of its gears.

There are two basic designs of gear pump: internal and external (Figure 1).  An internal gear pump has two interlocking gears of different sizes with one rotating inside the other.  An external gear pump consists of two identical, interlocking gears supported by separate shafts.  Generally, one gear is driven by a motor and this drives the other gear (the idler).  In some cases, both shafts may be driven by motors.  The shafts are supported by bearings on each side of the casing.

This article describes plastic gear pump in more detail.

There are three stages in an internal gear pump’s working cycle: filling, transfer and delivery (Figure 2).

As the gears come out of mesh on the inlet side of the pump, they create an expanded volume.  Liquid flows into the cavities and is trapped by the gear teeth as the gears continue to rotate against the pump casing.

The trapped fluid is moved from the inlet, to the discharge, around the casing.

As the teeth of the gears become interlocked on the discharge side of the pump, the volume is reduced and the fluid is forced out under pressure.

No fluid is transferred back through the centre, between the gears, because they are interlocked.  Close tolerances between the gears and the casing allow the pump to develop suction at the inlet and prevent fluid from leaking back from the discharge side (although leakage is more likely with low viscosity liquids).

External gear pump designs can utilise spur, helical or herringbone gears (Figure 3).  A helical gear design can reduce pump noise and vibration because the teeth engage and disengage gradually throughout the rotation.  However, it is important to balance axial forces resulting from the helical gear teeth and this can be achieved by mounting two sets of ‘mirrored’ helical gears together or by using a v-shaped, herringbone pattern.  With this design, the axial forces produced by each half of the gear cancel out.  Spur gears have the advantage that they can be run at very high speed and are easier to manufacture.

Gear pumps are compact and simple with a limited number of moving parts. They are unable to match the pressure generated by reciprocating pumps or the flow rates of centrifugal pumps but offer higher pressures and throughputs than vane or lobe pumps. External gear pumps are particularly suited for pumping water, polymers, fuels and chemical additives. Small external gear pumps usually operate at up to 3500 rpm and larger models, with helical or herringbone gears, can operate at speeds up to 700 rpm. External gear pumps have close tolerances and shaft support on both sides of the gears. This allows them to run at up to 7250 psi (500 bar), making them well suited for use in hydraulic power applications.

Since output is directly proportional to speed and is a smooth pulse-free flow, external gear pumps are commonly used for metering and blending operations as the metering is continuous and the output is easy to monitor. The low internal volume provides for a reliable measure of liquid passing through a pump and hence accurate flow control. They are also used extensively in engines and gearboxes to circulate lubrication oil. External gear pumps can also be used in hydraulic power applications, typically in vehicles, lifting machinery and mobile plant equipment. Driving a gear pump in reverse, using oil pumped from elsewhere in a system (normally by a tandem pump in the engine), creates a motor. This is particularly useful to provide power in areas where electrical equipment is bulky, costly or inconvenient. Tractors, for example, rely on engine-driven external gear pumps to power their services.

External gear pumps can be engineered to handle aggressive liquids. While they are commonly made from cast iron or stainless steel, new alloys and composites allow the pumps to handle corrosive liquids such as sulphuric acid, sodium hypochlorite, ferric chloride and sodium hydroxide.

What are the limitations of a gear pump?

External gear pumps are self-priming and can dry-lift although their priming characteristics improve if the gears are wetted.  The gears need to be lubricated by the pumped fluid and should not be run dry for prolonged periods.  Some gear pump designs can be run in either direction so the same pump can be used to load and unload a vessel, for example.

The close tolerances between the gears and casing mean that these types of pump are susceptible to wear particularly when used with abrasive fluids or feeds containing entrained solids. External gear pumps have four bearings in the pumped medium, and tight tolerances, so are less suited to handling abrasive fluids.  For these applications, universal gear pump are more robust having only one bearing (sometimes two) running in the fluid.  A gear pump should always have a strainer installed on the suction side to protect it from large, potentially damaging, solids.

Generally, if the pump is expected to handle abrasive solids it is advisable to select a pump with a higher capacity so it can be operated at lower speeds to reduce wear.  However, it should be borne in mind that the volumetric efficiency of a gear pump is reduced at lower speeds and flow rates.  A gear pump should not be operated too far from its recommended speed.

For high temperature applications, it is important to ensure that the operating temperature range is compatible with the pump specification.  Thermal expansion of the casing and gears reduces clearances within a pump and this can also lead to increased wear, and in extreme cases, pump failure.

Despite the best precautions, gear pumps generally succumb to wear of the gears, casing and bearings over time.  As clearances increase, there is a gradual reduction in efficiency and increase in flow slip: leakage of the pumped fluid from the discharge back to the suction side.  Flow slip is proportional to the cube of the clearances between the cog teeth and casing so, in practice, wear has a small effect until a critical point is reached, from which performance degrades rapidly.

Gear pumps continue to pump against a back pressure and, if subjected to a downstream blockage will continue to pressurise the system until the pump, pipework or other equipment fails.  Although most gear pumps are equipped with relief valves for this reason, it is always advisable to fit relief valves elsewhere in the system to protect downstream equipment.

The high speeds and tight clearances of external gear pumps make them unsuitable for shear-sensitive liquids such as foodstuffs, paint and soaps.  Internal gear pumps, operating at lower speed, are generally preferred for these applications.

What are the main applications for gear pumps?

External gear pumps are commonly used for pumping water, light oils, chemical additives, resins or solvents.  They are preferred in any application where accurate dosing is required such as fuels, polymers or chemical additives.  The output of a gear pump is not greatly affected by pressure so they also tend to be preferred in any situation where the supply is irregular.

Summary

An external gear pump moves a fluid by repeatedly enclosing a fixed volume within interlocking gears, transferring it mechanically to deliver a smooth pulse-free flow proportional to the rotational speed of its gears.

External gear pumps are commonly used for pumping water, light oils, chemical additives, resins or solvents.  They are preferred in applications where accurate dosing or high pressure output is required.  External gear pumps are capable of sustaining high pressures.  The tight tolerances, multiple bearings and high speed operation make them less suited to high viscosity fluids or any abrasive medium or feed with entrained solids.

External-gear pumps are rotary, positive displacement machines capable of handling thin and thick fluids in both pumping and metering applications. Distinct from internal-gear pumps which use “gear-within-a-gear” principles, external-gear pumps use pairs of gears mounted on individual shafts. They are described here along with a discussion of their operation and common applications. For information on other pumps, please see our Pumps Buyers Guide.

Spur gear pumps

Spur gear pumps use pairs of counter-rotating toothed cylinders to move fluid between low-pressure intakes and high-pressure outlets. Fluid is trapped in pockets formed between gear teeth and the pump body until the rotating gear pairs bring individual elements back into mesh. The decreasing volume of the meshing gears forces the fluid out through the discharge port. A relatively large number of teeth minimizes leakage as the gear teeth sweep past the pump casing.

Spur gear pumps can be noisy due to a certain amount of fluid becoming trapped in the clearances between meshing teeth. Sometimes discharge pockets are added to counteract this tendency.

Spur gear pumps are often fitted with sleeve bearings or bushings which are lubricated by the fluid itself—usually oil. Other fluids that lack oil’s lubricity generally demand more stringent pump designs, including locating bearings outside of the wetted cavities and providing appropriate seals. Dry-running bearings are sometimes used. The use of simply-supported shafts (as opposed to cantilevered arrangements seen in many internal gear designs) makes for a robust pump assembly capable of handling very thick liquids, such as tar, without concern for shaft deflection.

Helical gear pumps

Similar to the spur gear pump, the helical gear pump uses a pair of single- or double-helical (herringbone) gears. Helical gears run quieter than spur gears but develop thrust loads which herringbone gears are intended to counteract. These designs are often used to move larger volumes than spur gear pumps. Helical gears produce fewer pulsations than stainless gear pump as the meshing of teeth is more gradual compared with spur-gear designs. Helix angles run between 15 and 30°.

Both the helical and herringbone gear pumps eliminate the problem of trapping fluid in the mesh. These designs can introduce leakage losses where the teeth mesh, however, unless very tight tooth clearances are maintained. The higher manufacturing costs associated with herringbone gear pumps must be balanced against their improved performance.

Applications

External-gear pumps can pump fluids of nearly any viscosity, but speed must normally be reduced for thicker materials. A typical helical gear pump might run at 1500 rpm to move a relatively thin fluid such as varnish but would have to drop its speed nearer to 500 rpm to pump material as thick as molasses in July.

External-gear pumps generally are unsuited for materials containing solids as these can lead to premature wear, although some manufacturers make pumps specifically for this purpose, usually through the use of hardened steel gears or gears coated with elastomer. External-gear pumps are self-priming and useful in low NPSH applications. They generally deliver a smooth, continuous flow. In theory, at least, they are bi-directional. They are available as tandem designs for supplying separate or combined fluid-power systems.

These pumps are capable of handling very hot fluids although the clearances must be closely matched to the expected temperatures to insure proper operation. Jacketed designs are available as well.

External-gear pumps see wide applications across many industries: food manufacturers use them to move thick pastes and syrups, in filter presses, etc.; petrochemical industries deploy them in high-pressure metering applications; engine makers use them for oil delivery. They are used as transfer pumps. Special designs are available for aerospace applications. Pumps for fluid power will conform to SAE bolt-hole requirements.

External-gear pumps are manufactured from a variety of materials including bronze, lead-free alloys, stainless steel, cast and ductile iron, Hastelloy, as well as from a number of non-metals.

External-gear pumps can be manufactured as sanitary designs for food, beverage, and pharmaceutical service. The gears can be overhung, supported by bearings outside the housing with a variety of seals and packings available. Access to these internal pump components through a cover plate makes sanitizing straightforward. Gears are commonly manufactured from composites of PTFE and stainless steel as well as other plastics. Close-coupled and sealless designs are available.

External gear pumps are the least costly of the various positive-displacement pumps but also the least efficient. Pressure imbalances between suction and discharge sides can promote early bearing wear, giving them somewhat short life expectancies.

One general disadvantage that all heat preservation gear pump share over some other positive-displacement pump styles – vane pumps, for instance – is their inability to provide a variable flow rate at a given input speed. Where this is a requirement, a work-around is to use drives capable of speed control, though this is not always a practical solution.

Finally, while rotary, positive-displacement pumps are capable of pumping water, their primary application is in oils and viscous liquids because of the need to keep rubbing surfaces lubricated and the difficulty in sealing very thin fluids. For most applications where water is the media, the centrifugal, or dynamic-displacement pump, has been the clearer choice.